Fluorescence correlation spectroscopy, in its implementation in a microscope assemblage, is suitable for the investigation not only of non-directional molecular diffusion, but also of directional transport processes, such as particle flow, that may be overlaid on the diffusion. The concentration and diffusion coefficient of correspondingly labeled molecules, and the absolute value of their flow velocity can be derived from autocorrelation analysis of the fluorescence signal of the molecules. The radial symmetry of the focus about the optical axis makes it impossible, however, to determine the direction of the flow. (Publications: Brinkmeier (2001) in: Fluorescence Correlation Spectroscopy—Theory and Application, pp. 379–395, eds.: R. Rigler, E. Elson, Springer-Verlag, Heidelberg/Berlin; Dittrich & Schwille (2002): Anal. Chem. 74, 4472).
European Patent EP 0 941 470 discloses a system in which an FCS module is associated with an imaging scanning microscope. The FCS module is coupled directly onto the scanning microscope. The light for FCS examination is coupled out of the detection beam path of the scanning microscope and conveyed to the FCS module. Unequivocal detection of the diffusion direction is not possible with this system.